1. Field of the Invention
The present invention relates to a numerical controller capable of discontinuing the feed movement of a movable member of a machine tool upon the occurrence of an abnormality or trouble.
2. Discussion of the Prior Art
Generally, in numerical control machine tools, an emergency stop is performed upon detection of an abnormality or a trouble for preventing such a trouble during the automatic operation from resulting in an accident. That is, controllers for the numerical control machine tools check for the occurrences of troubles or abnormalities such as occur on drive units for driving servomotors, excessive movements of movable members beyond designed moving strokes and the like and upon detection of any of such troubles, execute an emergency stop in such a manner as to cancel the automatic operation and to break electric power to the drive units and the servomotors.
However, in the known machine tools, the emergency stop is performed not only upon the occurrence of a serious trouble leading to uncontrolled runs of the movable members, but also upon the occurrence of a minor trouble such as a voltage drop on a battery for a random access memory storing numerical control programs. This undesirably makes it difficult or time consuming to resume a discontinued machining operation after removal of any cause of the trouble, for the reasons mentioned below.
Although the distribution of pulses to feed servomechanisms is immediately stopped by the execution of an emergency stop, the feed servomechanisms and movable members of a machine tool driven thereby continue their movements due to their inertias thereafter. This makes a difference between the actual position of each movable member and the present position which is stored in the controller for use in numerical control operations. Therefore, after removal of any cause of the trouble, the operator has to do such troublesome procedures as to return the movable members to their original positions in a manual operation mode and to initialize or reset present position registers in the controller, for subsequent resumption of the automatic machining operation.